1. Field of the Invention
The present invention relates to a radio frequency identification (RFID) system, and more particularly, to an improved dual antenna coil antenna and signal processing RFID interrogator or reader for inductively coupling to a transponder to extract data from the transponder. A magnetic field emitted by the reader to energize the transponder is generated by currents flowing through two essentially identical but spaced apart antenna coils.
2. Related Art
In the automatic data identification industry the use of cooperative identification systems which may include an interrogator (also known as a reader) and a transponder (also known as a tag), has grown in prominence as a way to track objects and/or data regarding an object to which the transponder is affixed. A transponder generally includes a semiconductor memory, in which digital information may be stored. Using a technique known as inductive coupling, a transponder provides the stored data to an interrogator in response to an electromagnetic field that is generated by the interrogator. This type of inductively coupled identification system is very versatile. The transponders may be passive, in which they extract their power from the electromagnetic field provided by the interrogator, or active, in which they include their own power source. The passive transponders can be either “half-duplex” or full-duplex” transponders, which can be manufactured in very small, lightweight, and inexpensive units. The interrogator-transponder systems can be made to operate in a wide range of frequencies, from kilohertz to gigahertz. The interrogator may be portable and powered by a small battery, or fixed and powered by a battery or AC power.
In view of these advantages, inductively coupled identification systems are used in many types of applications in which it is desirable to track information regarding a moving or inaccessible object. Various applications may include asset and inventory control, access control, security, and transportation applications such as vehicle toll collection, parking, and fleet management. Another application is to affix transponders to animals in order to provide information such as their health, behavior, or location. One method of attaching the transponder is to implant the transponder within the animal. For example, the transponder may be implanted beneath the skin of the animal or the transponder may be designed such that, when swallowed, it remains in the stomach or digestive tract of the animal. Passive transponders are uniquely suited for this type of application because they do not require an internal power source such as a battery that can wear out.
The inductively coupled identification system may utilize an interrogator that generates through an antenna coil an electromagnetic field for inductively coupling to a transponder. The transponder may be passive and have a memory device coupled to an inductive antenna coil that serves both as the antenna and inductive power supply to draw power from a generated electromagnetic field to supply power to the transponder's electrical circuits. One method of providing data to the interrogator is for the transponder to retransmit the identification data to the interrogator. This approach requires the use of transmission and reception circuitry in both the interrogator and the transponder. Alternatively, because it is desirable to miniaturize the transponder, it is beneficial to eliminate as many parts in the transponder as possible. Thus, another method of providing the data to the interrogator is to provide a variable load within the transponder. To decode the data, the interrogator measures the power output of the interrogator and loading by the transponder. The modulated power signal is decoded to separate the data element for later digital interpretation.
A drawback of conventional inductively coupled identification systems is that the inductive coupling between the transponder's inductive antenna coil and the electromagnetic field, generated by the interrogator's field antenna coil, may depend on the relative distance between the interrogator's field antenna coil and the transponder's inductive antenna coil. If the distance between the interrogator's field antenna coil and the transponder's inductive antenna coil are minimized, then inductive coupling is maximized. However, if the distance is relatively far then inductive coupling is negligible and the inductive coupling is less effective. Accordingly, it would be desirable to provide an interrogator which increases the effective range for reading the transponder.
In Conventional inductively coupled identification systems the interrogator is unable or not designed to pick up disturbances in the magnetic field caused by conductive objects other than transponder tags. For example, an animal or human walking near the antenna coils may magnetically react to the magnetic field produced by the interrogator. However, conventional RFID systems lack the sensitivity and/or circuitry to detect any magnetic disturbances. Accordingly, it would be desirable to provide an interrogator which can detect conductive objects other than transponder tags.